Question

Python Programming

Given an integer k and a string s, find the length of the longest substring that contains at most k distinct characters.

For example, given s = "abcba" and k = 2, the longest substring with k distinct characters is "bcb".

Answer 1

I explain you the procedure through the given example: s= abcba and k=2

1. in the first iteration we check the whole string "abcba", as it contains more than k distinct elements we continue
2. in the second iteration we check the first 4 and the last 4: abcb and bcba , as both of these contain more than k distinct elements we continue
3. in the third iteration we check the first 3 and the next 3 and then the next 3: abc , bcb , cba  , as bcb contains 2 distinct elements which is <=k, we have found our result.

we have used set data structure to find the number of distinct elements.

A set contains only unique elements i.e. if we add abcba to a set , the set will contain a,b,c

then by finding the length of the set we can find the number of distinct elements.

PROGRAM:

#asking user to enter string
st = input("Enter a string: ")
n=len(st)

#asking user to enter value of k
k= int(input("Enter value of k: "))

#flag variable
f=0

for i in range(1,n+1): #iterate from 1 to length of string

    for j in range(i):  #iterate from 0 to i

        if len(set(st[j:n-i+j+1]))<=k:   # if number of distinct elements in substring <=k we have found the result
            r=st[j:n-i+j+1]    #recuired substring
            f=1
            break
    if f==1:
        break
print("substring: "+r)

OUTPUT 1:

Enter a string: abcba
Enter value of k: 2
substring: bcb

OUTPUT 2:

Enter a string: aabacbebebe
Enter value of k: 3
substring: cbebebe